Feb 17 (Reuters) - U.S. stocks were set to open lower on Friday, after a record-setting few days on Wall Street, as investors await clarity on economic policy and ahead of a long weekend.

The S&P 500 has risen about 5 percent so far in 2017, with the Dow Jones Industrial Average up 4 percent, mainly on signs of an improving economy and promises by President Donald Trump to cut corporate taxes and reduce financial regulations.

Now, with a strong fourth-quarter earnings season mostly complete, many investors say they need concrete signs of progress from Trump to justify more gains.

In his first solo news conference on Thursday, Trump again offered little details on his policy plans.

"Tax reforms are going to be as complicated as the repeal and replace (of the Affordable Care Act) and the markets may well think that the timing of the tax reform is going to be dragged out," said Art Hogan, chief market strategist at Wunderlich Equity Capital Markets in New York.

The lack of key economic data and a long weekend due to the Presidents Day holiday on Monday is also likely to keep investors from taking new positions, with trading volumes likely to be thin, on Friday.